Metadata

Many other DAM systems place resources on a tree type structure, as with a typical file system on a PC.
This works to a point, but it only allows each resource to be classified in one way.
For example, a resource taken in Africa can be placed in an 'Africa' folder.
If this also needs to be classified in a different way, e.g. by subject, emotion, project, size and so on, it cannot simultaneously exist in a different folder without duplication.

Using metadata as the primary organisation method allows resources to be classified in many different ways simultaneously, to the point that a static tree structure becomes obsolete.
For example, a photo resource could be classified as taken in Africa but also that it contains a photo of a boy, who is happy, and is making eye contact (via Subject, Emotion etc. metadata fields).
This allows searching via many different possible criteria.

Searching therefore becomes the primary method of finding resources.
This way users will become used to searching first and foremost and will not attempt to find resources on a tree type structure first of all.

Crucially, Collections are not a means of organising your resources.
Assuming that Collections are how you organise resources is quite a common mistake that newcomers make.
Think of Collections more like user bookmarks or favourites.
Metadata (and descriptive fields) are how you should organise/categorise resources.
The exception to this is the "Featured Collections" area which displays Collections which have been made public then added to a theme category - this is intended only as a sort of showcase area for your most used and most popular resources and not for organising all your resources.